This invention relates to a high-frequency differential amplifier stage comprising a first transistor and a second transistor whose emitters are coupled to each other and are connected to a first current source. The bases are connected to input terminals for receiving input signals and the collectors are connected to the emitters of a third and a fourth transistor, respectively. The third and fourth transistors have their collectors connected to power-supply terminals for connection to a supply-voltage source via a first and a second load resistor, respectively. The bases of the third transistor and the fourth transistor are connected to the collectors of the fourth and the third transistor, respectively. The invention also relates to a multi-stage amplifier comprising such a differential amplifier stage.
Such an amplifier stage is described in the U.S. Pat. No. 4,703,284. The amplifier stage forms the input stage of an amplifier which further comprises an output stage cascaded with the input stage. Further, the amplifier has a gain which can be made higher than unity for a maximum operating frequency of the amplifier, for example 1 GHz, which closely approximates the intrinsic cut-off frequency of the transistors used, while it has a high gain for frequencies situated in the central part of its operating frequency range. The input stage is a tuned amplifier which is capable of amplifying small signals and which has a substantial gain at frequencies situated in the central part of its operating frequency range, and which has a slightly higher maximum gain at frequencies situated in the upper part of its operating frequency range.